The development of reversibly binding radioligands for imaging brain proteins in vivo, such as enzymes, neurotransmitter transporters, receptors and ion channels, with positron emission tomography (PET) is keenly sought for biomedical studies of neuropsychiatric disorders and for drug discovery and development, but is recognized as being highly challenging at the medicinal chemistry level. This article aims to compile and discuss the main considerations to be taken into account by chemists embarking on programs of radioligand development for PET imaging of brain protein targets.

The imaging of translocator 18 kDa protein (TSPO) in living human brain with radioligands by positron emission tomography (PET) has become an important means for the study of neuroinflammatory conditions occurring in several neuropsychiatric disorders. The widely used prototypical PET radioligand [¹¹C](R)-PK 11195 ([¹¹C](R)-1; [N-methyl-¹¹C](R)-N-sec-butyl-1-(2-chlorophenyl)-N-methylisoquinoline-3-carboxamide) gives a low PET signal and is difficult to quantify, whereas later generation radioligands have binding sensitivity to a human single nucleotide polymorphism (SNP) rs6971, which imposes limitations on their utility for comparative quantitative PET studies of normal and diseased subjects. Recently, azaisosteres of 1 have been developed with improved drug-like properties, including enhanced TSPO affinity accompanied by moderated lipophilicity. Here we selected three of these new ligands (7–9) for labeling with carbon-11 and for evaluation in monkey as candidate PET radioligands for imaging brain TSPO. Each radioligand was readily prepared by ¹¹C-methylation of an N-desmethyl precursor and was found to give a high proportion of TSPO-specific binding in monkey brain. One of these radioligands, [¹¹C]7, the direct 4-azaisostere of 1, presents many radioligand properties that are superior to those reported for [¹¹C]1, including higher affinity, lower lipophilicity, and stable quantifiable PET signal. Importantly, 7 was also found to show very low sensitivity to the human SNP rs6971 in vitro. Therefore, [¹¹C]7 now warrants evaluation in human subjects with PET to assess its utility for imaging TSPO in human brain, irrespective of subject genotype.